Motors are used widely in industrial apparatuses, and for many such motors, a method is used in which forced cooling is provided by a cooling apparatus in order to mitigate heat generation by the motor. There is, for example, a method in which a cooling fan is attached to a motor to supply cooling air to the motor and thus to forcibly cool the motor. In another method, in which no cooling fan is used, the flow path for the cooling medium is formed in the motor such that the cooling medium in the flow path is circulated by a pump while the cooling medium is cooled by an oil cooler in order to forcibly cool the motor. An apparatus that is an accessory for a motor for forcibly cooling the motor is hereinafter generally referred to as a cooling apparatus.
In many industrial apparatuses, a motor operating condition set for the purpose of maximizing the production efficiency applies a heavy load to the motor; thus, when an operating condition is set that applies, to a motor, a load that is equal to or greater than the cooling capacity of the cooling apparatus, the motor may generate heat. When dust adheres to a cooling apparatus or a flow path is clogged, the cooling capacity may be degraded, which may cause the motor to generate heat under an operating condition that would not originally have been a problem. When a motor generates heat rapidly or the temperature of the motor remains high after heat generation, the motor cannot deliver its original performance. Furthermore, the life of the motor may be reduced and thereby unexpected failure may be caused. Hence, when a motor has generated heat, it is necessary to detect the heat generation early and take an appropriate countermeasure that corresponds to the cause of the heat generation.
With regard to a method of estimating the temperature change amount of a motor, Patent Literature 1 describes a method in which the amount of temperature increase resulting from acceleration and deceleration of a main shaft motor and the amount of temperature increase resulting from cutting performed during a certain rotation are estimated and the operating condition is changed in accordance with the cause of the temperature increase.